Fluid flow sensing and control systems are included in various systems, devices, and environments. For example, many aircraft rely on accurate airflow sensing and control for various performance and environmental functions, such as engine starting, equipment cooling, and cockpit and cabin environmental control. No matter the particular end-use, typical flow sensing and control systems include one or more flow transducers to sense the mass flow rate of the fluid being controlled, and supply a signal representative of the sensed flow to a control law. The control law may then command one or more flow control devices, such as one or more valves, to a appropriate position to achieve a desired fluid flow rate.
One particular type of flow transducer that has been and continues to be used is a thermal mass flow transducer. A typical thermal mass flow transducer includes a pair of temperature sensing elements, a heater, and a control circuit. One of the temperature sensing elements is heated by the heater, whereas the other is not. The control circuit is coupled to the temperature sensing elements and the heater, and supplies current to the heater to maintain a constant temperature difference between the temperature sensing elements. The heater current needed to maintain the constant temperature difference is also representative of the fluid mass flow rate. To generate and supply the heater current, the control circuit typically includes a DC-type current driver, which is usually a power transistor-based driver.
The thermal mass flow transducer described above works reasonably well, but does exhibit certain drawbacks. For example, of the energy dissipated by the thermal mass flow transducer, about 40-70% is by the DC-type current driver, and only about 30-60% is by the heater. The relatively large energy dissipation by the DC-type current driver results in a significant energy waste. Moreover, this energy is dissipated in the form of heat, which may be conducted to the non-heated temperature sensing element, resulting in reduced flow sensing accuracy.
Hence, there is a need for a thermal mass flow transducer that dissipates relatively low amounts of wasted energy and/or exhibits increased accuracy, as compared to presently known transducers. The present invention addresses one or more of these needs.